The activated sludge process is widely used as a treatment method of various types of organic wastewater such as sewage and industrial wastewater, even among biological treatment methods which biologically treat organic wastewater including organic substance because of good quality of treated water and its advantage of easy maintenance. However, the volume loading of BOD (organic substance expressed as biochemical oxygen demand) upon an activated sludge tank which conducts an activated sludge treatment is about 0.5 to 0.8 kg/m3/day. As a result, in order to operate at a high load, there is a problem in that it becomes necessary to enlarge the activated sludge tank and a wide setting area is necessary. In addition, most of the BOD which is assimilated by bacteria which forms the activated sludge is used as a respiratory substrate for bacteria and is decomposed into carbon dioxide and water, and one part is used for propagating bacteria, specifically, about 20 to 40% of the BOD which is absorbed by the bacteria is used for bacterial growth. That is, because around 20 to 50% of the BOD which is absorbed by activated sludge is transformed into bacteria, treatment of organic wastewater by the activated sludge has a problem where the bacteria which are grown using the BOD as their substrates are ejected as excess sludge.
Consequently, a fluid bed method in which supports are added to an activated sludge tank is known. According to the fluid bed method, because bacteria are retained on the supports, the concentration of bacteria within the activated sludge tank is increased and high loading treatment where the volume loading of BOD is about 3 kg/m3/day becomes possible. However, under the fluid bed method, the amount of excess sludge is more than the usual activated sludge method, more specifically, about 30% of the BOD biologically decomposed will be changed to excess sludge.
As a result of this, a biological treatment method of organic wastewater which establishes a secondary treatment tank which preserves sessile protozoa as a latter step of a primary treatment tank (activated sludge tank) is known (for example, patent document 1). According to the method disclosed in the patent document 1, by applying a high BOD loading to the primary treatment tank, the growth of sessile protozoa is suppressed and the aggregation of bacteria is prevented. Activated sludge treated water which includes dispersed bacteria and flows out from the primary treatment tank is introduced to the secondary treatment tank. Because sessile protozoa which prey on dispersed bacteria are preserved in the secondary treatment tank, dispersed bacteria are preyed upon by the sessile protozoa and excess sludge is reduced and the biologic community is aggregated. Therefore, aggregations of microorganisms (sludge) which have good sedimentation properties are formed in the secondary treatment tank and the effluent discharged from the secondary treatment tank is separated into a solid and liquid and clear treated water is obtained.
In this way, by combining the activated sludge tank operated at a high load with a minute organism preserving tank which preserves sessile protozoa, it is possible to reduce the amount of excess sludge at a high load, and more, to obtain clear treated water. As a result, various refinement methods concerning the activated sludge process which uses an activated sludge tank and a minute organism preserving tank are proposed. For example, in patent document 2, a biological treatment device is arranged with a bait breaking down tank between the activated sludge tank and the minute organism preserving tank. In the device disclosed in patent document 2, the flocculated bacteria are dispersed by performing ultrasonic treatment etc. in the bait breaking down tank so that they are easily consumed by the protozoa which are preserved in a latter tank, that is the minute organism preserving tank.    [Patent document 1] Japanese Laid-Open Patent Publication No. S55-20649    [Patent document 2] Japanese Laid-Open Patent Publication No. S57-74082